A recent study by the American Heart Association has noted that cardiovascular diseases, such as heart attacks and strokes, are the leading cause of death in the Western world. Traditionally, x-ray angiography has been the primary method of diagnosing the presence and significance of arterial stenoses, the underlying cause of cardiovascular disease. In the United States alone over 1.4 million diagnostic x-ray angiography procedures are performed each year. In addition to diagnostic x-ray angiography, widely-used interventions such as percutaneous transluminal coronary angioplasty (PTCA) are performed under x-ray angiographic guidance. Accordingly, x-ray angiography is by far the most widely used technique to diagnose and treat cardiovascular disease.
While extremely useful, x-ray angiography involves ionizing radiation and requires contrast agents which are known to cause renal injury. (See, Aspelin et al, Nephrotoxic Effects in High-Risk Patients Undergoing Angiography. New England Journal of Medicine. 348(6): 491-99, the entire content of which is expressly incorporated hereinto by reference.) Therefore, an alternative approach to x-ray angiography which does not involve contrast agents or ionizing radiation is desirable.
In this regard, magnetic resonance imaging (MRI) is a well known diagnostic technique that does not rely on ionizing radiation and has thus been proposed as an alternative to x-ray angiography. Existing interventional MRI techniques, however, are characterized by slow movie frame rates, poor spatial resolution, and the need to track the complex 3-dimensional location of catheters using traditional 2-dimensional MR images. To date, interventional MRI has not achieved clinical application.
A new class of magnetic resonance images has recently been described which is based on a previously unrecognized spin state known as global coherent free precession (GCFP). (See, Rehwald et al. Non-Invasive Cine Angiography by Magnetic Resonance Global Coherent Free Precession, Nature Medicine 2004; 10(5), and U.S. patent application Ser. No. 10/449,252 filed on May 30, 2003, the entire content of each being expressly incorporated hereinto by reference.) The use of GCFP allows the acquisition of images depicting blood flow. In brief summary, protons within moving blood are “tagged” every few milliseconds as the blood flows through a slice in space. Simultaneously, previously tagged blood is maintained in the GCFP state which allows acquisition of consecutive movie frames as the heart pushes blood out of the excitation slice. Body tissue surrounding the moving blood is never excited and therefore remains invisible. The processes used to “tag” blood as it flows through the slice are the traditional combinations of radiofrequency (RF) and gradient waveforms to achieve spatially-selective proton excitation.